If we're going to be this picky, 29% of the US identifies as Republican; 25% of that supports gay marriage; therefore, 7.25% of the country are Republicans who support gay marriage. That is slightly more than half the number of African Americans.
However, the general thrust of the point is the more important one than these petty calculations, I think. He's right that we are not supportive of anti-gay sentiment on this forum. There are no comparable positions that Democrats support for which we are equally as intolerant; that is, there is no liberal position which can in and of itself get you banned from this site. As such, a discussion between Republicans and Democrats would naturally weed out a huge number of Republicans just based on this policy position alone, and what you'd be left with is... a large pool Democrats, libertarian leaning Republicans, and moderate Republicans.
Which is, as it turns out, almost exactly what we see.